Astronomy Unit 5
Astronomy Unit 5 Astronomy 1020
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This 17 page Study Guide was uploaded by Emily Mason on Friday April 15, 2016. The Study Guide belongs to Astronomy 1020 at Clemson University taught by Dr. Flower in Winter 2016. Since its upload, it has received 125 views.
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Date Created: 04/15/16
1 Astronomy Unit 5 Expanding Universe (Chapter 31): Hubble Flow- Expansion of the universe Helps us interpret radial velocities Cosmological Redshifts- Redshifts caused by expansion General relativity predicts that as photons travel across an expanding universe, their wavelengths stretch along with space Longer wavelengths = redshifts The farther away a galaxy lies, the longer light from it had to travel before reaching us, and the more its light was cosmologically redshifted More distant galaxies have greater observed radial velocities Hubble time (page 248)- Distance divided by velocity is the inverse of the Hubble constant: D/V=1/H oroH o1 The distance of any individual galaxy divided by its velocity is H -1 o -1 If we use 71km/s/Mpc for H , toen H o equals 14 billion years—this is the same for all galaxies For nearby galaxies, the smaller distances offset the smaller velocities so that the ratio D/V is the same It is a measure of the age of the universe Compare to the expansion of the universe (page 250-251)- Figures 31.6 and 31.3 2 Hubble’s Law- Distance is proportional to velocity Hubble found that redial velocity increased with distance The last “bridge” across the abyss of space D represents distance, V represents the measured radial velocity, and H 0 represent the constant of proportionality V= H o D The constant of proportionality is the Hubble Constant and we must calibrate it from galaxies with known distances Once we know H , ohe measurement of the radial velocity of a galaxy gives its distance H oas an average value of 71 km/s/Mpc This behavior suggests a uniform expansion of the universe The most distant galaxies have the highest radial velocities The expansion of the universe causes the observed galaxy radial velocities (Hubble flow) The greater the separation for a given velocity, the longer the galaxies have been separating Cosmic microwave background (page 253) Also called CMB Background radiation Cosmic refers to the radiation coming from everywhere in space Its discovery is a major confirmation of the big bang model The temperature is exactly what astronomers predict to be due to the redshifted radiation from a hot early universe Cosmic temperatures less than 5 K to 10 K resulted from the cooling expansion of a once hot universe Hydrogen and Helium Abundances (page 260) The mass fraction of the visible baryonic matter is about ¾ H and ¼ He 3 Stars and supernovae create all of the heaviest elements The tiny fraction of heavy elements represents their total production in stars over the history of the universe Nucleosynthesis in stars and supernovae cannot account for all of the observed H and He Stars do not produce hydrogen Hydrogen is primordial—created early in the universe before stars formed 4 Stars do produce helium-4 ( He) in main-sequence hydrogen burning and in hydrogen nuclear shell sources in evolved stars Nuclear reactions convert the He into heavier elements No more than a few percent of the observable He in the universe is made during stellar evolution—He must be primordial as well The metallicities of old stars are near zero, reflecting the lack of stellar evolution and the lack of heavy element synthesis - Their He abundances are between .23 and .25, less than that of the youngest stars - The measured amount of abundance (since it is not equal to 0) represents its primordial abundance The universe contains measurable amounts of He as well Observers find only about 20 atoms of He for every million H atoms Only a small fraction of both isotopes of He makes it to the interstellar medium Temperature of the Universe (page 253) 4 Critical Density (page 249) A measure of the amount of matter necessary to barely keep the universe open, p c It represents the density of the matter necessary for the universe to remain barely open If the overall density of the universe were equal to the critical density, the universe would be flat If the density were greater than the critical density, the universe would be closed If the density were less than the critical density, the universe would be open A young universe means a time of faster expansion, requiring a greater p tc stop the collapse Ω m p/p c If omega is greater than 1, the universe has more mass than it needs to close the universe If omega is less than 1, the universe doesn’t contain enough mass to stop the expansion A Flat Universe (page 257-258) This figure shows the different peaks in temperature differences at different angular scales on the sky The different peaks provide info on the geometry of the universe, its density, and the amounts of dark matter 5 The first peak tells us that the universe is flat - The curvature of space can change the apparent sizes of the imprinted hot and cold spots - A flat universe will allow the light from these spots to travel in straight lines and keep the observed size equal to the actual size Galaxy Formation with non-baryonic matter seeds (page 256 and 281) The slight temperature differences we see now reflect density variations in the distribution of matter in the early universe The inferred clumps of matter in the early universe represent the “seeds” for galaxy formation Density enhancements occurring during the earliest moments of the expansion will attract more mass and collapse into galaxies and walls of baryonic matter we see in the distribution of galaxies Seeds- sites where matter can collect in a relatively short time Seeds must form concentrations of mass before baryonic matter decouples Seeds would need to be massive enough to attract the baryonic matter once it decoupled from the radiation field Seeds would need to have decoupled before baryonic matter so that the seeds would have time to form large, massive structures The seeds must be some form of non-baryonic dark matter Energy Densities (page 258) There are three types of energy densities in the universe: 1. Ω m o Contribution due to mass is omega=.27 2. Ω ^ o Since the CMB observation shows that the universe is flat, omega here must be= 0.73 3. Ωc o The value of omega= zero for a flat universe, greater than zero for an open universe, and less than zero for a closed universe 6 o The CMB observation shows that the universe if flat, =0 Astronomers were reluctant to consider dark energy, but: This changed when astronomers discovered that the expansion of the universe was speeding up instead of slowing down as expected Compare Visible (baryonic) and Dark Matter to Critical Density (page 258) Dark and visible matter can’t account for more than 27% of all matter in the universe 4% is visible matter The universe is flat and the density of matter is below the critical density - The expansion of the universe is not slowing down and there is an energy of empty space that has properties of antigravity - Since energy and mass are equivalent, the mass equivalent of this energy is enough to make the universe flat Type Ia Supernovae Observations (page 258) These supernovae are so bright that they are visible in galaxies with large redshifts They are always the result of the explosion of a white dwarf They are fainter than predicted by a purely decelerating universe They reveal that galaxies have separated farther than we thought in the last 5 billion years Expansion of the Universe Begins to Accelerate- (page 259) Age of the Universe—13.7 billion years old 7 The Early Universe Humans have learned to harness four sources of nature: 1. Electromagnetic force- we make use of chemical reactions between atoms and molecules that are bound to this force. 2. Gravitational force- used by producing mechanical energy and electricity from falling water 3. Strong nuclear force- generate electrical power 4. Weak nuclear force- also generate electrical power Elementary Particles of Nature (pg.266) o Elementary means the most fundamental. Elementary particles appear inseparable and have no obvious internal structure o Leptons- the six types of elementary particles Some of these include the electron, the muon, and the tau They all have negative charge The muon and tau have a larger mass than an electron, and therefore are less stable and decay into electrons and neutrinos Each lept0n has an antiparticle o Antiparticle- have the same mass, lifetimes, and other attributes of their lepton particle except that their electrical charge is opposite. o Quarks- small fundamental constitutes of protons and neutrons. Atomic particles that are made up of quarks are divided into two groups 1: Baryons- consists of protons and neutrons, and are made up of three quarks. They make up the normal matter in the universe 2: Mesons- consist of quark-antiquark pairs (yes, this is apparently a real thing). Play an important role with the forces between particles o Overall, particles in nature consist of one or more of the 12 elementary particles (6 quarks and 6 leptons) and their 12 antiparticles Exchanging Particles (pg. 267) o Forces in nature arise from the exchange of mediators, like a photon, that “carry” the force at finite speeds o Gravitons- the mediators of the electromagnetic and gravitational forces o Gluons- along with mesons, they are exchange particles for the strong nuclear force Mediate the force between neutrons and protons Exchanged within atomic particles binding together quarks that make up the particle Confines quarks to distances within 10^-17cm of each other’s o Mediators of the weak nuclear force consist of three particles: W , W , and Z 0 o Standard Model- the theoretical basis for understanding elementary particles and their interactions. Includes the four forces. 8 Unified- term physicists use when they cannot tell the four forces apart from each other when they are at large energies o The name of the unified electromagnetic and weak forces, for instance, is called electroweak force (very creative.) The grand unification theories (GUT) are theories that describe the merger of the electroweak force and the strong force o Predict an imbalance of matter and antimatter through the decay of massive x particles More massive than a proton, are unstable, and decay into combination of quarks and leptons and their antiparticles Theory of Everything- the label given to the unification of all four forces into one superforce Diagram of the Temperatures at which Forces Combine Annihilation (369)- a collision between a particle and its antiparticle result in the complete annihilation of bot particles o Appears in the form of gamma rays Pair Production- Pair of photons are created when the energy of the two photons are greater or equal to 2???????? , or two times the speed of light. Threshold Temperature- the minimum temperature necessary for the production of a particle-antiparticle pair of a given mass o As the temperatures go below the threshold, pair production stops but annihilation continues Annihilation Catastrophe- the destruction of all matter through annihilation 9 Inflation field- the field produced as the universe expanded and cooled that infused energy into the expansion o Acts like a negative gravitational field, and has the property of banishing at extremely high temperatures o Occurred when the strong force separated from the electroweak force All the energy of the field was released through this occurring This energy then creates more matter in the universe o Lasted only 10^-34 seconds inflation- term for the rapid expansion of the universe produced from the appearance of the inflation field and has the properties of an explosion; hence, the Big Bang o Before The Horizon Problem (pg. 271) The Flatness Problem (pg. 273) o Our horizon is not large enough for us to easily notice the curved surface of the Earth. Inflation expansion does this to the universe. Inflation increases the scale of any pre-existing curvature to such a great extent that we perceive the universe to be flat. Quantum Theory o Properties such as position and velocity do not have definite values, but assigns them probabilities that look like waves when on a graph (see page 274) o Used to determine the origin of the universe Quantum cosmology (pg. 274)- 0ne application of quantum physics to determine the creation of the universe 10 o the quantum nature of fields suggest that the initial conditions that led to the universe were chaotic o Can sometimes predict the conditions necessary for inflation, therefore being able to predict what it will do in the future Multiverse- bubbles of energy (universes) that randomly emerge from the vacuum The first microsecond of the Universe o Existed with one unified force operating o The gravitational force and GUT governed particular interactions for 10^-34 seconds. o Then, the universe was so hot that the strong, electromagnetic, and weak forces were unified as a single force o After the GUT died out, inflation occurred and then ended when the strong force froze out o Soon entered an electroweak age with more matter than antimatter (the universe has remained this way to today) During this phase, the energies of the quarks were so great that quarks and gluons mixed freely in quark-gluon plasma o The quark age began at 10^-12 seconds, and the electroweak and weak forces froze out, and all four forces acted independently Set the stage for nucleosynthesis and the creation of the elements Neutron to Proton Ratio (276) o Particles interacted through the strong and weak nuclear forces o Just as many neutrons existed as protons because the temperature of the universe was then below their threshold temperatures o When temperatures drops, they were able to form into hydrogen o As the temperature continued to decrease, an imbalance of neutrons and protons occurred: the protons became greater and greater o After 3 seconds, the temperature became too low for weak force reactions and the ratio froze out at n/p = 1/6 Primordial Nucleosynthesis o At one hundred seconds, the universe consisted of a ratio of neutrons to protons of 1:7 o Nucleosynthesis of light elements could only occur when the temperatures dropped below a billion K, when photodisintegration becomes ineffective. (the universe reaches this temp at 100 seconds) o Neutrons began forming with protons to dorm deuterium ( an isotope of hydrogen) o Deuterium could form with another neutron to form tritium, which could then form HELIUM (formed in the plasma stage) pg. 277-278 11 o This was a time of a frenzy of neutron reactions, but this only took a matter of minutes o Once this frenzy was over, the universe consisted mainly of helium-4 o When all of the neutrons were used up, more production of helium was possible Atoms (279) o Scattering- a process by which free electrons and photons collide and “bounce” off each other, moving off in different direction from which they approached each other. o During the plasma age, electrons were so dense that photons could only travel a few centimeters at most before scattering This caused the universe to be opaque o The density and energy of electrons was decreased over time, so they could no longer stay free of nuclei—therefore, atoms formed. This occurred when the universe was 380,000 years old and the temperature was about 3000k Decoupling (279)- free electrons combining with nuclei so that photons could not scatter off them. AKA the cessation of the interactions between matter and radiation. Since decoupling the universe has expanded a thousand fold and has cooled to 2.726K. The universe then became transparent because photons were free to travel greater distances Diagram of the History of the Universe 12 Galaxy Formation—CLUMPS (280) o The entire universe is not exactly at the same temperature o This results in a clumpy universe We see evidence of this through microwave background radiation and decoupling o Temperature difference represent density variations in matter at decoupling o Clumps cannot immediate collapse into galaxies or other structures They experience the gravitational attraction of their particle-rich surroundings that tends to prevent the collapse Rapid collapse only occurs when the contrast between the density of a clump and its surroundings is a factor of 2 clumps: 2 average density of the universe. o ***Observations of redshifts of radio galaxies and quasars suggest the galaxy formed within one billion years of inflation*** Seeds of Growth (281) o Seeds are sites about which matter can collect in a relatively short time The theory is that in this way, baryonic matter would not have to wait until its own density was twice that of its surroundings, underlying the structure of these seeds Seeds would need to have decoupled from the radiation field before baryonic matter so that the seeds would have time to form large, massive structures Must be some form of non-dark matter o Cold dark matter (CDM)- being considered by theorists as a type of non-baryonic matter Main characteristic is that they move too slowly to smooth out any density variations in the universe Usually massive, lumber particles like quark nuggets (huge concentrations of quarks), the axion (a low mass but slow moving particle that may have been copiously produced when quarks merged to form protons and neutrons), and the neutralino (a particle with the mass of a large atom) Future of the Universe (282) o Galaxies will continue to separate from each other forever and evolve o Spirals and irregular galaxies have little life left They will shine brightly for a short time then explode and disappear as black holes or neutron stars. o Massive stars will have faded away while low mass main sequence stars steadily burn their hydrogen fuel The lowest mass stars will take 100 billion years to burn their hydrogen 13 They will briefly brighten as red giants, becoming the brightest stars in the galaxy, but eventually fade as cooling white dwarfs o At some point, the universe will fade into darkness Dead planets will orbit dead suns and dead suns orbit dead galaxies This will only last about 10^28 years The future of matter in the universe is to eventually decay into electrons, positrons, and photons o When matter begins to decay, dead galaxies will become visible again All the galaxies will form into gigantic black holes through collision Eventually the black holes will evaporate in a burst of gamma rays o The universe comes to an end in a final burst of energy Sample Quiz 1. The expanding universe causes: a. Cosmological redshifts b. Extra Info: Recall that when stars are moving away from us, their spectra exhibit redshifts - their spectral lines are shifter toward long wavelengths. The shift is due to their motion around the galaxy relative to ours. All galaxies, on the other hand, are "moving" away from us. But the motion is not their own motion in the universe. If that were true, then we are at the center of the universe. Instead, the fabric of space of which the galaxies are part, is expanding. The redshifts we observe are due to the cosmos expanding; hence the term cosmological redshifts. 2. The Hubble time: a. Is a measure of the age of the universe b. Extra Info: The Hubble time is 1/H o H othe Hubble constant) measures the current expansion rate of the universe. If the rate was unchanged since the beginning of time, the its inverse would be the age of the universes. The Hubble constants is the time it takes for a galaxy to move away from us and be at the measured distance. It is essentially distance/velocity and this is just time. Recall the equation distance = velocity X time that you can use to determine the time it would take you to travel a certain distance at a certain velocity or speed. The same with galaxies; H =odistance / velocity. The Hubble constant is a "measure of time" because the universe expanded faster in the past. the gravitational attraction between galaxies is slowed down the expansion. Thus, the Hubble time is greater than the age of the universe. See Figure 31.3 in the text. 14 3. About how old is the universe: a. 13.7 billion years 4. Approximately when did the expansion of the universe begin to accelerate? a. About 5 billion years ago b. Extra Info: See figure 31.15 5. What did the observation of the brightness of the Type Ia supernovae tell us about the universe? a. The expansion of the universe is accelerating b. Extra Info: see Figure 31.15 6. A closed universe is one that: a. Will contract sometime in the future b. Extra Info: The gravitational attraction between galaxies is so strong (due to high density of matter in the universe) in a closed universe slows down the expansion and eventually stops it. 7. What kind of universe has an average density equal than the critical density? a. Flat b. Extra Info: The critical density is the division between a higher density that causes the universe to collapse upon itself and a lower density that allows the universe to easily expand. The critical density is just high enough to allow the universe to expand forever. A flat universe is one that has a density of matter equal to the critical density. 8. What observation showed us that the universe was flat: a. Minute temperature variations in the cosmic microwave background radiaton b. Extra Info: The temperature variations in the cosmic background radiation shows us how the different parts of the universe differed in temperature and the sizes of different regions of different temperature. In a flat universe, the spacing between the hot and cold spots remains the same over the age of the universe. We can calculate the spacing using the general theory of relativity. Open or closed universes give different spacing, which can also be calculated. Figure 31.13 shows hot the observations favor a flat universe. 9. This drawing shows curves for empty, open, closed, and flat universes. Which curve corresponds to a universe without matter? a. The letter “a” b. Extra Info: See Figure 31.6 15 10.One major significances of the measured cosmic background radiation is that: a. It tells us that the universe was hotter in the past b. Extra Info: The basis for this is that the universe was hot at it inception and emitted the entire spectrum of radiation just like the Planck curves of starYou know that, the Planck curve for a cools star is shifted to longer wavelengths. The cosmic background radiation we observe today started out as the radiation of a hot universe. Since it emitted a Planck curve spectrum and since the cosmic background radiation is a Planck curves shifted to very long wavelength (cm), its peak gives the current temperature of the universe. See Figure 31.8. 11. The temperature of the universe today is 3.575 K a. FALSE b. IT IS ACTUALLY 2.726 K (See figure 31.8) 12. Which list below includes 3 of the 4 known forces in nature: a. Strong nuclear, gravity, electromagnetic 13. The universe increased in size extremely rapidly during: a. Inflation b. Extra Info: The discovery of the rapid expansion of the universe near its inception occurred when the United States was experiencing a tremendous period of inflation. Thus, the term inflation for this period. 14. What process that occurred in the early universe creates particles from photon collisions: a. Pair production b. Extra Info: This is energy being converted to mass: E = mc . The early universe was nearly pure photons and their collisions produced many particles. 15. The fact that neutrons decay into protons in a short time causes a. Less helium produced in the early universe compared to their neutrons not decaying. b. Extra Info: Since the universe needs neutrons to make helium (= 2 n + 2p), the decay of neutrons after an age of 3 seconds, reduced the number of neutrons relative to protons (hydrogen). Fewer neutrons limited the number of helium nuclei made through the fusion with protons. The remains protons are hydrogen nuclei. 16. What observation indicates that there was very limited time for nucleosynthesis in the early universe: a. The small abundances of heavy elements in the universe b. Extra Info: Heavy nuclei are the result of nuclear reactions between lighter nuclei. Hydrogen to helium to carbon etc. These reactions need higher and higher temperatures. Since the no carbon was made early in the universe, the universe did not have enough time at high enough temperatures to produce carbon and other heavy elements. In fact, the temperature of the universe was decreasing 16 during the time of nuclear reactions. The cooling happened fast enough to prevent the creation of heavy elements. 17. At about what time after the creation of the universe was helium produced? a. A few minutes b. After about 3 minutes, the temperature was too cool for nuclear reactions of any sort 18. Why were only light elements created during the early minutes of the universe? a. Expansion cooled the universe too quickly for nuclear reactions to create heavier elements. 19. What was the mass fraction of hydrogen in the universe before galaxies formed? a. 0.75 20.The point of time when the universe became transparent is called decoupling a. True 21. The formation of “seeds” allowed for: a. The formation of galaxies b. Extra Info: Seeds are concentrations of dark matter in the very early universe. See Figure 32.14. Since dark matter exerts a gravitational attraction, the normal (baryonic) matter naturally condensed around the concentrated dark matter. 22. What percentage of the matter in the universe is composes of material like stars, dust, planets (visible baryonic matter): a. 4.4% b. Extra Info: 27% is the combination of normal matter and dark matter. Of this 4.4% is normal matter. The other 73% is dark energy 23. Which of the following lists correctly gives the contributions of the energy density due to dark energy in our universe: a. .73 24. Which of the following correctly describes the expansion history of the universe: a. Instantaneous expansion followed by a decelerating expansion followed by an accelerating expansion b. Figure 31.15 25. Which of the following lists gives the correct sequence (some events may be missing from the list) of events in the early universe, arranged from the earliest times to the most recent? a. Helium production (primordial nucleosynthesis), decoupling, galaxy formation. b. Figure 32.12 17 26. Which of the following most correctly describes our universe a. The universe is 13.7 billion years old, is currently expanding, underwent an episode of rapid expansion called inflation, and will expand forever
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